The present invention relates to a dispersing agent for suspension polymerization of vinyl compounds, and more particularly to a vinyl alcohol polymer-based dispersing agent comprising a vinyl alcohol polymer which has conjugated double bonds in its molecule and the fatty acid ester units of which are more randomly distributed as compared with conventional vinyl alcohol polymer-based dispersing agents.
Industrial production of vinyl resins has been generally performed by batchwise suspension polymerization wherein a vinyl monomer is dispersed into an aqueous medium in the presence of a dispersing agent in a polymerization vessel and, after adding an oil-soluble polymerization initiator to the vessel, the temperature is elevated to polymerize the monomer. In recent years, in order to shorten the polymerization time required for a batch of polymerization to thereby enhance the productivity, a polymerization vessel equipped with a reflux condenser has been used to accelerate removal of polymerization reaction heat, or a hot water charging polymerization method is proposed wherein a hot aqueous medium is charged to shorten the time for elevating the temperature up to polymerization temperature.
For example, in the production of vinyl chloride resins, however, the use of a polymerization vessel equipped with a reflux condenser has the problem that wet foam and dry foam vigorously generate because the pressure in the vicinity of the reflux condenser lowers by condensation of a vinyl chloride monomer gas. The wet foam is caused by polyvinyl alcohol used as a dispersing agent for the polymerization and is mainly composed of water, and the dry foam is mainly composed of vinyl chloride resin particles. These foams generate mainly in the middle to last stage of polymerization. If wet foam and dry foam generate, a problem arises that the productivity lowers by decrease in effective volume in the polymerization vessel. In case of dry foam, further problem may arise that fouling adheres to the reflux condenser to result in difficulty in control of temperature in the polymerization vessel. If no reflux condenser is used, the polymerization time is of course long, so the productivity is low.
As a countermeasure therefor, it is proposed to add a dispersing agent such as a partially hydrolyzed polyvinyl alcohol to a polymerization system in the stage where the proportion of removal of polymerization reaction heat by the reflux condenser to the overall reaction heat is low (e.g., JP-A-2-180908 and JP-A-3-212409). However, this proposal has the problem that generation of dry foam becomes vigorous, so the bulk density of the produced vinyl chloride resins is easy to be lowered.
Also proposed are methods of polymerization wherein partially hydrolyzed polyvinyl alcohol resins having specific degrees of hydrolysis and specific degrees of polymerization are added before starting the polymerization or when the polymerization conversion falls within a specific range (e.g., JP-A-55-137105, JP-A-7-179507, JP-A-7-53607 and JP-A-7-18007). However, these methods have the defect that the polymerization time is long since no reflux condenser is used and, therefore, the productivity is low.
JP-A-10-1503, JP-A-11-116630 and JP-A-2001-122910 propose methods of polymerization wherein a reflux condenser is used and partially hydrolyzed polyvinyl alcohol resins having specific degrees of hydrolysis and specific degrees of polymerization are added before starting the polymerization or when the polymerization conversion falls within a specific range. Since these methods use a polymerization vessel equipped with a reflux condenser, the productivity is high. However, defoaming effect to suppress dry foam is not sufficient, and these methods still have problems such as fouling buildup. In light of advance in technology, further improvement is desired.
On the other hand, it is reported that vinyl polymers having excellent physical properties are produced by using a polyvinyl alcohol resin having conjugated double bonds in its molecule as a dispersing agent for suspension polymerization of vinyl compounds (e.g., JP-A-8-269112 and JP-A-8-283313). The introduction of conjugated double bonds into polyvinyl alcohol resins is achieved by heat treatment of polyvinyl alcohol resins or carbonyl group-containing polyvinyl alcohol resins whereby fatty acid such as acetic acid is eliminated to produce double bonds. However, in general, conventional methods for the production of polyvinyl alcohol resins having conjugated double bonds require a heat treatment operation at about 150° C. for 5 to 6 hours and accordingly have a problem of increase in production cost. Conventional methods also have the problems that insoluble matter may be produced since there are many occasions of contacting oxygen in production steps and, in addition, the degree of randomness in the sequence distribution of remaining fatty acid ester units such as acetoxy groups does not rise. Further, vinyl polymers obtained by suspension polymerization of vinyl compounds using known dispersing agents comprising polyvinyl alcohol resins having conjugated double bonds in the molecule have a wide particle size distribution. The reason is considered to be that the distribution of conjugated double bonds introduced by conventional methods is not uniform.
JP-A-8-283313 discloses that the polyvinyl alcohol resin having conjugated double bonds in its molecule disclosed therein as a dispersing agent can suppress foaming in suspension polymerization of vinyl compounds. Foaming can be suppressed to some extent, but the proposed dispersing agent has the problem that since the function as a protective colloid is insufficient, coarse particles are produced when hard vinyl chloride resins are produced.
Therefore, a dispersing agent for suspension polymerization which has a satisfactory ability as a surface active agent and can suppress foaming has been demanded.
It is an object of the present invention to provide a polyvinyl alcohol-based dispersing agent which can suppress foaming in suspension polymerization of vinyl compounds.
A further object of the present invention is to economically provide a dispersing agent for suspension polymerization of vinyl compounds.
A still further object of the present invention is to provide a dispersing agent which can provide vinyl polymers having excellent properties and a uniform particle size distribution.
Another object of the present invention is to provide a process for economically preparing a polyvinyl alcohol resin which is useful as a dispersing agent for suspension polymerization of vinyl compounds and provides vinyl polymer particles having excellent properties without generating foaming during the suspension polymerization.
These and other objects of the present invention will become apparent from the description hereinafter.